How can multi-level page tables save memory?

Hello, everyone. I have been looking at the multi-level page table of the operating system recently, but I still have some doubts about the multi-level page table.

the first-level page table covers all the virtual address space, and some of his table items are not used, so there is no need to create a second-level page table for it. The second-level page table is only created when needed. Where are the used / unused first-level page tables?

if there is a secondary page table, is the top-level page table in memory, and the corresponding page tables (secondary page tables) are also in memory, while those unused page tables are on disk?

Aug.04,2021
MySQL Query : SELECT * FROM `codeshelper`.`v9_news` WHERE status=99 AND catid='6' ORDER BY rand() LIMIT 5
MySQL Error : Disk full (/tmp/#sql-temptable-64f5-1b36844-2c025.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
MySQL Errno : 1021
Message : Disk full (/tmp/#sql-temptable-64f5-1b36844-2c025.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
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